CORNISH: Yeah, OK, let's get real. It's the time of year many homeowners build on, build out or build up. But what if there's just no room for your house to grow?

SHAPIRO: You could do what they do in London - dig down. That's what NPR's Robert Smith told us about a few months ago when he sent us this report about the city's subterranean building boom.

JOHN COYLE: You'd be surprised what's underground (laughter).

ROBERT SMITH, BYLINE: Above us is a street of narrow London townhouses. Down here, it is the Wal-Mart of basements.

This is huge. I mean, look at how tall the ceilings are down here.

It is still under construction. It has been for nine months. The contractor, John Coyle, walks me through what is to come.

COYLE: I just wanted to give you an idea of the scale of it.

SMITH: Bedroom, bathroom, another bedroom, utility room, a home theater.

COYLE: So this will be - this will be the cinema room here. This will be a big projector and a screen on that wall.

SMITH: I have literally been in movie theaters smaller than this room.

COYLE: Yeah, it's amazing. Yeah, come down and...

SMITH: I'm guessing reclining leather chairs...

COYLE: Yeah, everything.

SMITH: Popcorn machine.

COYLE: Everything.

SMITH: It's not just that the English love to sit in dark, damp spaces. This basement boom is happening because wealthy Londoners feel like they have no other option. The most fashionable neighborhoods here feature these really narrow houses. And historic restrictions make it hard to add anything that you can see from the street. And so the engineers start digging. Some residential streets in London have had dozens of basements dug out. There's even a name for them, iceberg houses, bigger below the ground than above.

COYLE: Where we're standing now, it wouldn't be unusual to have another staircase down to a swimming pool.

SMITH: It is a simple calculation. London real estate is so expensive that adding any extra space is worth the money. This basement right here costs $1.8 million to build. It will probably add $3 million to the value of the home. Usually, you can double your money. Of course, things do go wrong. Tim Chapman is a subterranean engineer with Arup consultants. He's dug some of the biggest holes in the city. And he says once you go down in an old place like London, you can find anything - underground rivers, sewer pipes, Roman ruins, old train tunnels.

Is there any dirt left under London?

TIM CHAPMAN: You're quite right. I mean, there's a huge amount of things underground in London. Every decade, we add a new tube line. People are putting in new fiber optic cables. And no one takes any of these things away. So it's a form of pollution.

SMITH: Find a human bone or a piece of pottery, and suddenly your new basement project can take twice as long. And then there are the neighbors. If you're rich enough to build a million-dollar basement, your neighbor is probably rich enough to get a lawyer. Chapman is often hired as an expert consultant in these cases. And he says in such tight quarters like London, things do go wrong.

CHAPMAN: If you dig a basement, it is inevitable that there'll be some movement of the property next-door. So people who say, I'm going to cause no effect whatsoever, you can't dig a basement without causing no effect whatsoever.

SMITH: In fact, last year, a massive house collapsed in the city. It came out afterwards that they were adding a cinema, a home gym and a wine room. There was even a debate in the House of Lords over the epidemic of digging, as they called it. Some of the richer folks wanted a moratorium. But the contractor John Coyle says once he starts building a luxury basement, many of the neighbors want one too. Right across the street, Phoebe Dickerson has been watching this basement construction for the last nine months. And I asked her, would she ever do it?

PHOEBE DICKERSON: I think it's quite expensive. I'm not sure if I'm in the market for that.

SMITH: They told us that for every dollar you spend on it, you get $2 back.

DICKERSON: Really? Well, I'm still not in the market for it - maybe one day.

SMITH: (Laughter).

Hey, if you want to move up in this world, you've got to dig down. Robert Smith, NPR News, London. Transcript provided by NPR, Copyright NPR.